The present invention relates to an improvement in a lithium cell, or lithium battery, comprising lithium metal or an alloy of lithium as the cathode-active material and a polymeric solid material containing a lithium salt as the ion-conductive electrolyte filling the space between the anode and cathode. More particularly, the invention relates to an improvement in a lithium cell, by which the cell can be imparted with a high ion-conductivity of the electrolyte and large discharge capacity as well as high stability of the cell performance by repeating cycles of discharging and recharging so that the cell can be used as a secondary or rechargeable battery.
Cells or batteries of which the cathode-active material is lithium metal or an alloy mainly composed of lithium are generally called a lithium cell or battery and the application fields of lithium cells are rapidly growing by virtue of the outstandingly long life as the most characteristic feature of lithium cells. A lithium cell has a basic structure comprising a cathode of a cathode-active material which is metallic lithium or an alloy of lithium, an anode of an anode-active material, for which various kinds of materials are known to be suitable depending on the desired characteristics of the cell, and an ion-conductive electrolyte composition filling the space between the cathode and anode.
Various types of electrolyte compositions are known as a material filling the space between the cathode and anode in a lithium cell, of which the most conventional is a highly flowable solution prepared by dissolving a lithium salt in an organic solvent. Electrolyte compositions in a gel-like form are also known as prepared by adding a gelation agent to an organic solution containing a lithium salt.
Along with the rapid progress in recent years in the electronics industry, in particular, toward the so-called microelectronics, lithium cells are also required to be smaller and smaller in size, thickness and weight as is typically the case when the cell is used as a backup power source for computer memories where the cell should be built in the instrument and integrated with the electronic circuit. Needless to say, lithium cells using a liquid or gelled electrolyte composition can hardly meet the requirement because a separator between the electrodes is indispensable in such a lithium cell in order to prevent direct contacting of the cathode and the anode in addition to the problems relative to the narrow temperature range for use of the cell and leakage of the liquid electrolyte composition when corrosion takes place in the casing or collector plates of the cell.
With an object to solve the above mentioned problems, attempts have been made to use an electrolyte in a solidified state and various kinds of polymeric solid electrolyte compositions are proposed therefor including combinations of a lithium salt with a polymer such as poly(ethylene oxide), poly(ethylene imine), polyphosphazene and the like. These polymeric solid electrolyte compositions, however, have several disadvantages that the ionic conductivity thereof is not high enough, that they are relatively poor in the mechanical strengths, that no reliable stability can be obtained in contacting between the electrodes and the electrolyte and so on so that lithium cells prepared with such a polymeric solid electrolyte composition have no practically satisfactory discharge characteristics with a relatively rapid decrease in the performance of the cell in the lapse of time resulting in limited durability. Therefore, no practically usable lithium cells have yet been obtained by using such a polymeric solid electrolyte composition.